Variable resistance materials are promising active materials for next-generation electronic storage and computing devices. A variable resistance material possesses two or more states that differ in electrical resistance and can be programmed back and forth between the states by providing energy to induce an internal chemical, electronic, or physical transformation of the material that manifests itself as a change in resistance of the material. The different resistance states can be associated with different data values and used as memory states to store or process data.
Phase change materials are a promising class of variable resistance materials. A phase change material is a material that is capable of undergoing a transformation, preferably reversible, between two or more distinct structural states. The distinct structural states may be distinguished on the basis of, for example, crystal structure, atomic arrangement, order or disorder, fractional crystallinity, relative proportions of two or more different structural states, or a physical (e.g. electrical, optical, magnetic, mechanical) or chemical property. In a common embodiment, the two or more distinct structural states include differing proportions of crystalline phase regions and amorphous phase regions of the phase change material, where the phase-change material is reversibly transformable between the different states. In the crystalline state, the phase change material has lower resistivity; while in the amorphous state, it has higher resistivity. Continuous variations in resistivity over a wide range can be achieved through control of the relative proportions of crystalline phase regions and amorphous phase regions in a volume of phase-change material. Reversibility of the transformations between structural states permits reuse of the material over multiple cycles of operation.
Typically, a variable resistance device is fabricated by placing the active variable resistance material, such as a phase change material, between two electrodes. Operation of the device is effected by providing an electrical signal between the two electrodes and across the active material. In a common application, phase-change materials may be used as the active material of a memory device, where distinct data values are associated with the different structural states and each data value corresponds to a distinct resistance of the phase-change material. The different structural states employed in memory operation may also be referred to herein as memory states or resistance states of the phase-change material. Write operations in a phase-change memory device, which may also be referred to herein as programming operations, apply electric pulses to the phase-change material to alter its structural state to a state having the resistance associated with the intended data value. Read operations are performed by providing current or voltage signals across the two electrodes to measure the resistance. The energy of the read signal is sufficiently low to prevent disturbance of the structural state of the phase-change material.
In order to expand the commercial opportunities for phase-change memory, it is desirable to identify new phase-change compositions, device structures, and methods of programming that lead to improved performance. A key performance metric for phase-change memory is programming current. In order to reduce power consumption in commercial devices, it is desirable to minimize the current needed to program the device. The structural transformations that occur between amorphous, crystalline, and mixed amorphous-crystalline states require the input of energy. Most commonly, the energy is thermal in nature and produced by the Joule heating that accompanies the flow of electric current through the active phase-change material or surrounding resistive layers in electrical and thermal communication with the active phase-change material. Programming a phase-change memory device necessitates the production of a sufficient amount of Joule heating to increase the temperature of the phase-change material to above a threshold temperature (either the crystallization temperature or melting temperature, see below).
To minimize power consumption, it is necessary to design the phase-change device to insure that a given programming current maximizes the amount of thermal energy available for programming. Several strategies have been proposed for improving the power efficiency of phase-change devices. One strategy is to construct a device structure in which the phase-change material is surrounded by highly thermally insulative materials. The rationale of this strategy is to retain as much of the Joule heat that develops during programming as possible in close proximity to the phase-change material. Surrounding insulators prevent thermal dissipation of Joule heat to the adjacent device structure and improve utilization of the heat for programming purposes by localizing it in the vicinity of the phase-change material for a period of time sufficient to complete programming.
A second strategy is to place a resistive heater in close proximity to the phase-change material. This strategy recognizes that Joule heating scales with the power dissipated by the programming current, where power dissipation increases for a given current as the resistivity of a material increases. By placing a series resistive heater adjacent to a phase-change material, significant heating in the local vicinity of the phase-change material can occur at reduced programming currents.
A third strategy is to minimize the volume of phase-change material that undergoes a structural transformation during programming. The volume of phase-change material that undergoes a structural transformation may be referred to herein as the programmed volume of the phase-change material. This strategy recognizes, from a thermodynamic standpoint, that the heat needed to effect a structural transformation scales with the volume of material undergoing the transformation. By reducing the programmed volume of a phase-change material, a reduction in programming current results.
One approach for minimizing the programmed volume is to modify the device structure to reduce the area of contact between the phase-change material and the electrode that delivers the programming current. The use of conductive sidewall layers as electrodes, for example, permits electrical communication with the phase-change material to occur through the edge of a conductive layer. Since the edge thickness corresponds to the thickness of the layer and since layer thickness can be controlled at angstrom-scale dimensions, small contact areas (including areas having sublithographic dimensions) can be achieved. As an alternative, small dimensions can be achieved by patterning of a conductive layer.
A second approach for minimizing the programmed volume is to adopt a confined cell device geometry. In the confined cell geometry, a narrow (preferably sublithographic) opening is formed in a surrounding (e.g. dielectric) layer and the phase-change material is formed in the opening using a conformal technique such as chemical vapor deposition or atomic layer deposition. The narrow physical dimensions of the phase-change material necessarily lead to a reduction in programming volume.
A third approach for minimizing the programmed volume is to incorporate a breakdown layer into the structure. The breakdown layer is one of the simpler approaches for reducing the programmed volume. A breakdown layer is a highly resistive or insulating layer formed between the phase-change material and the electrode of the device that delivers the programming current (referred to hereinafter as the programming electrode). When initially formed, the breakdown layer is intact and creates a resistive or insulating barrier between the programming electrode and phase-change material. The thickness of the breakdown layer, however, is kept sufficiently small to permit rupturing of the breakdown layer upon application of a critical voltage between the programming electrode and counterelectrode of the device. At the point of rupture, the breakdown layer is punctured and a conductive pathway from the programming electrode to the phase-change material develops. The advantage of the breakdown layer is that the lateral dimension of the puncture is typically much smaller than the lateral dimension of the adjacent phase-change material. The net result is a device structure in which the most of the phase-change material is shielded from the programming electrode by the resistive or insulative non-punctured portion of the breakdown layer and only a small area region of the phase-change material is in a low resistance path of current flow delivered by the programming electrode. The punctured region represents a high conductivity region through which current provided by the programming electrode preferentially flows. The net effect is a constriction of current flow to the punctured region and a reduction in the portion of the phase-change material influenced by the current.
From a processing standpoint, the breakdown layer approach for reducing programming current is the simplest to implement. The fabrication of breakdown devices only requires the inclusion of a single thin layer deposition step in the process flow. Many dielectrics known to be compatible with phase-change compositions function as breakdown layers, so inclusion of a breakdown layer adds no significant processing uncertainties. The cost, in terms of time and materials, needed to incorporate a breakdown layer is also minimal.
A drawback that has been identified for breakdown layers is inconsistency in the location of the punctured region. Small changes or differences in processing conditions (e.g. temperature, sputtering intensity, impurities), device structure (e.g. localized thickness or composition of constituent layers, electrode composition, device geometry, particulate contamination), or operating conditions (e.g. width or amplitude of programming pulse) can influence placement of the punctured region. In one device, the puncture may be located near the center of the breakdown layer, while in another device, the puncture may be located near the edge of the breakdown layer. The size of the puncture may also vary and the possibility of multiple punctures also exists. Differences in the location, size or number of punctures may lead to variability in programming current, endurance and other operating characteristics of the device. Variability in performance from device-to-device is especially problematic in memory device arrays. In order to standardize programming and reading protocols, it is desirable to have consistent performance for all devices in the array.